It's "part of Office," but as far as I've been told in getting a license at work, it's not even part of Office 2007 Professional or Ultimate. The 2007 version is allegedly sold only stand-alone. If that's not true I'd appreciate being corrected.

An anonymous reader writes: August 2, 2007 (The INTERNET). The KDE Community is happy to announce the immediate availability of the first Beta release for KDE 4.0. This release marks the beginning of the integration process which will bring the powerful new technologies included in the now frozen KDE 4 libraries to the applications.
Almost two months after the foundations of KDE 4 have been laid with the first alpha, KDE enters the stage of a full freeze of the library interface. From now on, the applications will focus on integrating the new technology refined during the last months, and the library developers will try to fix all bugs found during this process. No new applications will enter the official KDE modules and usability and accessibility work is of course an ongoing process. In the following weeks KDE developers will be able to add features to their applications until the next beta is released and the application features will be frozen as well.Link to Original Source